1. Field of the Invention
This invention relates to a crystal growth method and, more particularly, to a method for growing crystals of a multicomponent compound semiconductor, for example, ternary and quaternary semiconductor compounds such as In.sub.1-x Ga.sub.x As, InAs.sub.x P.sub.1-x, In.sub.x-1 Ga.sub.x Sb, GaAs.sub.x P.sub.1-x, and In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y compounds, in a saturated growth solution thereof. This invention also relates to an apparatus for carrying out the above crystal growth method.
The new growth method and apparatus of this invention are effective for controlling the composition of the growth solution and, therefore, the composition of the grown crystals to a desired proportion.
2. Description of the Prior Art
Heretofore, various two component compound semiconductors (also known as binary semiconductors) have been widely used in the production of semiconductor devices. Recently, compound semiconductors containing three or more components or elements, generally referred to as multicomponent compound semiconductors, are used for the same purpose, since they have excellent electrical properties similar to or higher then those of the binary semiconductors. Typical of these compound semiconductors are ternary and quaternary semiconductors of Group III-V and Group II-VI elements of the Periodic Table, for example, In.sub.1-x Ga.sub.x As, InAs.sub.x P.sub.1-x, In.sub.1-x Ga.sub.x Sb, In.sub.1-x Al.sub.x As, GaAs.sub.x P.sub.1-x, Ga.sub.1-x In.sub.x P, Ga.sub.1-x Al.sub.x P, In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y, and the like.
In the production of these multicomponent compound semiconductors in the form of crystals, there are several problems to be solved. First, only limited types of the compound semiconductors can be prepared by the conventional methods, since the number of binary or ternary compounds usable as the substrate in the crystal growth applications are limited, and the resulting compound semiconductors must be lattice-matched to the substrate. Further, although the In.sub.1-x Ga.sub.x As, InAs.sub.x P.sub.1-x, In.sub.1-x Ga.sub.x Sb, GaAs.sub.x P.sub.1-x, In.sub.1-x Ga.sub.x As.sub.1-y P.sub.y and similar bulk single crystals can be conventionally produced by well-known methods such as the encapsulated Czochralski and gradient freeze methods, they contain an unfixed proportion or ratio of the elements constituting them: caused by compositional variations of the growth solution during the crystal growth step due to depletion of the solute elements in the solution essential to complete the compound semiconductor crystals. The saturated growth solution for use in the formation of the multicomponent compound semiconductor crystals is in the form of a solution or melt (hereinafter referred to as solution), and essentially consists of or contains two or more solute elements. In addition, it is difficult to obtain a thick homogeneous epitaxial layer of the compound semiconductors by means of a liquid phase epitaxy (LPE) process, because, as described above, solute elements in the growth solution having a finite or limited volume are depleted during the growth of the crystals.
The above problems will be now described with reference to the figures. FIG. 1 illustrates an X-Y compositional plane of InGaAsP quaternary compounds and FIG. 2 shows a phase diagram of quasi-binary system compounds.
As illustrated in the diagram of FIG. 1, the InGaAsP quaternary compounds ideally can possess any composition of four essential elements In, Ga, As, and P. Further, the InGaAsP compounds have three important areas, one of which is a high speed area A. The compounds belonging to this area will enable the production of semiconductor devices having a high speed response. The second area is a carrier confinement area B, in which area the compounds have an increased capability for uptaking carriers. The third area is a shorter wavelength area C, in which area the compounds are effective for the production of semiconductor devices capable of emitting visible radiation.
However, since the materials usable as the substrate in the production of the InGaAsP quaternary compounds are limited to only two types, namely, InP and GaAs (other binary compounds are not available), the scope of the resulting InGaAsP compounds is substantially outside the areas A, B, and C described above. In practice, possible InGaAsP compounds are indicated by two characteristic lines, a and b, since these can be lattice-matched to the substrates InP and GaAs, respectively. It is, therefore, desirable to provide bulk single crystals of the ternary compounds such as InGaAs, InAsP, InGaP, GaAsP, and the like: Since, if these crystals are available and usable as the substrate, an epitaxial layer of InGaAsP compounds with various lattice constants which covers the areas A, B, and C can be freely grown on the substrate. Further, if such InGaAsP compounds are available, they will remarkably increase the degree of flexibility and freedom in the design, fabrication, specification, and the like of the finally produced InGaAsP-based semiconductor device. Of course, these results are commonly applicable to all multicomponents compound semiconductors of Group III-V and Group II-IV elements in addition to the above-discussed InGaAsP compounds.
The reason why crystals, namely bulk crystals or epitaxial crystals of the multicomponent compound semiconductors have not yet been provided, in spite of the widespread demand by users', is apparent from FIG. 2 showing a phase diagasm of quasi-binary compounds AC and BC. In FIG. 2, it is assumed that AC and BC mean GaAs and InAs, respectively and therefore AC has a higher melting point than that of BC. It is apparent from a liquidus line l and a solidus line s that the composition X.sup.l of the growth solution is different from the composition X.sup.s of the crystals at the same growing temperature T.sub.G and, accordingly, if the growth solution used has a finite volume, the element A or Ga in the growth solution is gradually consumed and, therefore, the composition of the growth solution varies, as shown by arrow x in the line l. Further, accompanying the compositional variation of the growth solution, the composition of the growing crystals also varies, as indicated by arrow y in the line s. These results demonstrate that the use of the growth solution having a finite volume does not ensure the growth of crystals having a uniform composition, and necessarily results in bulk crystals or epitaxial crystals with compositional variation. Thus, it is conceived that homogeneous crystals of the multicomponents compound semiconductors will be produced when the high-melting compound AC or GaAs is supplied to the saturated growth solution under controlled conditions during the crystal growth step.
Recently, J. J. Daniele and A. J. Hebling have reported in J. Appln. Phys., 52 (1981) 4325 that very thick epitaxial layers (up to 600 .mu.m) of Al.sub.1-x Ga.sub.x As having a uniform composition could be grown by the Peltier-induced liquid phase epitaxy (LPE) process. In their work, an Al.sub.1-x Ga.sub.x As shell floating on the top of the growth solution was used as a source material, namely, material capable of supplying solute elements. However, the Daniele and Hebling method is not intended to control the supply of the solute elements into the growth solution, and no controllable methods of supplying solute elements into the growth solution have been developed as yet.